1. Field of Invention
The present invention relates to a switched-mode power supply, and in particular to a switched-mode power supply without an external inductor.
2. Related Art
A switched-mode power supply (SMPS) is a power supply employing the architecture of an inductor and a field effect transistor (FET) switch, which uses magnetic coils (inductor) as an energy storage device. This type of power supply can provide the highest power conversion efficiency (up to 97%), of all direct current conversion methods, and can improve the battery life of a portable product, thus prolonging the product's working time.
Since an SMPS uses an inductor as an energy storage element, an inductance value of the energy storage inductor will directly influence efficiency. A higher inductance value can decrease the ripple and hysteresis loss generated by an SMPS, and thus an inductor of 4.7 μF, 10 μF or above is generally used.
However, larger inductor occupies larger area of a circuit board, resulting in high cost. Accordingly, as in the current trend of integrating circuits, direct fabrication of an inductor into an integrated circuit (IC) would be an excellent choice. However, a very large circuit area will be occupied if an inductor of higher than 10 nF is fully-integrated in an IC, which is not consistent with a reasonable cost and also has a low quality factor. Therefore, whether in a buck converter, a boost converter, a buck boost converter, or a Cuk converter SMPS, the inductor is implemented off-chip.
In the SMPS, the switch of an MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) in a power stage is generally controlled by pulse width modulation (PWM) or pulse frequency modulation (PFM). The PWM method is changing the duty cycle of an MOSFET to adjust the output, without changing the cycle T of the MOSFET; and the PFM method is maintaining the duty cycle, but changing the cycle T to adjust the output. A dual-mode chip has also become available, which works in a PFM mode during low load, such as in waiting status, so as to reduce the operating frequency and thus decrease the power consumption; and is switched to operate in PWM mode during high load, so as to provide a high load current.
FIG. 1 is a schematic view of a conventional buck converter power supply 1, in which an external element 2 includes an inductor L1 and a capacitor C1 as energy storage elements, and circuit elements other than the external element 2 are generally disposed in an IC. A frequency compensation circuit 20 obtains a feedback voltage VFB via a voltage-dividing circuit formed of a resistor R1 and a resistor R2, and an error amplifier detects the change of an output voltage Vout according to a difference between the feedback voltage VFB and a reference voltage Vref, so as to generate an error signal, Vc. A comparator 14 compares the error signal Vc output from the frequency compensation circuit 20 with a triangle wave Vsaw, so as to generate a PWM output VDUTY. A driver embodied by a non-overlap clock generator 12 then generates two non-overlap clock signals according to the PWM output VDUTY, so as to control the upper and lower bridge switches P1 and N1 in a power stage 18, and generate an output current to charge and discharge the inductor L1 and the capacitor C1, thus supplying an output voltage Vout to a load RL.
In a current PWM modulator or PFM modulator, a comparator-based design is generally employed, in which the PWM modulator is generally a circuit including a comparator and a triangle wave; and the PFM modulator is a circuit including an SR latch circuit and a comparator. The inductance value of an external inductor L1 is correlated with the operating frequency of a power supply, and when the inductor L1 has a high inductance value, the power supply can operate at a lower frequency; on the contrary, when the inductor L1 has a low inductance value, the power supply needs to operate at a higher frequency.